a) Field of the Invention
The invention is directed to a method and arrangements for determining an at least approximately circular contacting surface of a deformed medium on a plane surface, in particular for verifying an applanation circle in an applanation tonometer.
b) Background Art
The change in the optical characteristics of a waveguide when contacting a medium is the subject of various publications, e.g. on substance analysis (EP 403 468, GB 2 156 970, DE 37 23 159). These arrangements, which are constructed as chemo-sensors, are concerned only with detecting or verifying various substances in the adjacent media, wherein a selective, permeable waveguide layer or film in which the substances to be verified are diffused or bonded to this film undergo changes in optical density or in the index of refraction so that changes in the phase, polarity or intensity of the penetrating light can be detected. Further, a known solution is concerned with the change in the above-mentioned physical characteristics brought about by a contacting surface of varying magnitude and is applied in ophthalmology for measuring the intraocular pressure of the human eye. It makes use of the same effective mechanism as that used in EP-OS 40 34 68 in that two vibrational modes which are oriented vertically to one another and in which different effective refractive indices act along the contacting surface with the aplanatic cornea of the eye are excited by coherent, linearly polarized light in a film waveguide. Phase differences accordingly result between the two modes and can be determined in various ways, e.g. in the form of a change in intensity when using a (linear) polarizer prior to a receiver.
However, a disadvantage in the known working principle consists in that the expenditure for determining the phase difference is still relatively high (lasers, polarizers and analysis of the change in intensity, costly adjustment of the polarizers), particularly for accurately determining the magnitude of the contact area.